Acrylonitrile-based precursor fiber for carbon fiber and method for production thereof

ABSTRACT

This invention relates to an acrylonitrile-based precursor fiber for carbon fiber which is prepared from an acrylonitrile-based copolymer containing 96.0 to 98.5% by weight of acrylonitrile units, and which is characterized by a tensile strength of not less than 7.0 cN/dtex, an elastic modulus in tension of not less than 130 cN/dtex, an iodine adsorption of not greater than 0.5% by weight based on the weight of the fiber, a degree of crystal orientation (π) of not less than 90% as determined by wide-angle X-ray analysis, and a degree of variation in tow fineness of not greater than 1.0%. This precursor fiber has a high strength, a high elastic modulus, a high degree of denseness, a high degree of orientation, and a low degree of variation in tow fineness, and can hence be used to form a high-quality carbon fiber inexpensively by oxidation for a shorter period of time.

TECHNICAL FIELD

This invention relates to polyacrylonitrile-based precursor fibers forcarbon fibers and a process for preparing the same.

BACKGROUND ART

Carbon fibers and graphite fibers (herein referred to collectively as“carbon fibers”) formed by using polyacrylonitrile-based fibers asprecursors have excellent mechanical properties and are hence beingcommercially produced and sold as fibrous reinforcements ofhigh-performance composite materials for use in aerospace applications,sports and leisure applications, and the like. Moreover, in recentyears, the demand for carbon fibers is growing in general industrialapplications such as automobile and marine applications and buildingmaterial applications. Thus, in order to enhance the performance of suchcomposite materials, inexpensive carbon fibers having high quality aredesired in the market.

In contrast to acrylic fibers for clothing use, acrylonitrile-basedfibers for use as precursors of carbon fibers are no more thanintermediate products for the formation of carbon fibers as finalproducts. Accordingly, it is not only desirable to provideacrylonitrile-based fibers capable of yielding carbon fibers havingexcellent quality and performance, but it is also very important thatthe acrylonitrile-based fibers have good stability during spinning ofprecursor fibers, exhibit high productivity in forming carbon fibers,and can be provided at low cost.

From this point of view, a large number of propositions have been madein order to provide acrylonitrile-based fibers capable of yieldingcarbon fibers having high strength and high elasticity. Thesepropositions include, for example, an increase in the polymerizationdegree of the copolymer, and a decrease in the content of copolymerizedcomponents other than acrylonitrile. As to the spinning method, dry-wetspinning is commonly employed.

However, when the content of copolymerized components other thanacrylonitrile is decreased, the solubility of the resulting copolymer insolvents is generally reduced. This not only detracts from the stabilityof the spinning solution, but also causes an extreme increase in theviscosity of the spinning solution, making it necessary to reduce thecopolymer concentration in the spinning solution correspondingly.Consequently, the copolymer shows a marked tendency toward precipitationand coagulation, so that the resulting fibers may frequently undergodevitrification or develop a large number of voids therein. Thus, thisproduction method cannot be regarded as a stable one.

Since the dry-wet spinning process comprises extruding a polymersolution through a nozzle into air and then passing it continuouslythrough a coagulating bath to form filaments, it is easy to obtain densecoagulated filaments. On the other hand, a decrease in the pitch ofnozzle holes will cause a problem in that adjacent filaments may adhereto each other. Thus, there is a limit to the number of nozzle holes.

Generally, an increased density of nozzle holes is advantageous for thelow-cost production of acrylonitrile-based precursor fibers.Accordingly, the wet spinning process is being employed, partly becauseit requires a relatively low cost of production equipment. However, theresulting filament tow generally include many broken filaments and muchfluff. Thus, the resulting precursor fibers have a low tensile strengthand a low elastic modulus, and the fiber structure of the precursorfibers is less dense and has a low degree of orientation. Consequently,the mechanical properties of the carbon fibers obtained by carbonizingthem are generally unsatisfactory.

For precursor fibers used to form high-quality carbon fibers, it is veryimportant that they are free of minute defects which will be responsiblefor breakage after they are converted to carbon fibers. In order tominimize such defects, it is necessary that the precursor fibers have ahigh tensile strength and a high elastic modulus, their fiber structurebe highly dense, the copolymer be highly oriented in the direction ofthe fiber axis, and the degree of variation in tow size be small.

For example, Japanese Patent Laid-Open No. 214518/'83 makes mention ofthe denseness of the fiber structure while employing the wet spinningprocess. As measures of the denseness, the amount of iodine adsorbed andthe thickness of the skin layer to which iodine is adsorbed are definedtherein. However, the precursor fiber thus obtained has a low density asdemonstrated by an iodine adsorption of about 1-3% by weight, and alsohas a low tensile strength and a low elastic modulus. Consequently, itis very difficult to produce a carbon fiber having high quality.

On the other hand, Japanese Patent Laid-Open No. 35821/'88 discloses aprecursor fiber which has been prepared by the dry-wet spinning processand which has a highly densified surface structure. Moreover, JapanesePatent Laid-Open Nos. 21905/'85 and 117814/'87 disclose precursor fiberswhich have also been prepared by the dry-wet spinning process and whichhave a high tensile strength and a high elastic modulus and comprise acopolymer highly oriented in the direction of the fiber axis. Althoughan improvement in the quality of the resulting carbon fibers can beachieved by using these precursor fibers, their productivity is lowowing to the use of the dry-wet spinning process. Moreover, the fibersprepared by dry-wet spinning have a smoother surface as compared withthe fibers prepared by wet spinning. The former fibers exhibit goodbundling properties, but also have several disadvantages in that theytend to fuse together in the oxidation step and in that they tend toshow poor spreadability in the formation of a sheet-like prepreg.Furthermore, the polymers used in these inventions practically have anacrylonitrile content of not less than 99.0% by weight. Accordingly,from the viewpoint of the stability of the spinning solution and thetendency of the copolymer toward precipitation and coagulation, theseprocesses are unsatisfactory for the stable preparation of a precursorfiber.

In order to obtain a precursor fiber having a densified surfacestructure while employing the wet spinning process, pressurized steamdrawing has been investigated as a drawing method for achieving a higherdraw ratio.

For example, Japanese Patent Laid-Open No. 70812/'95 discloses aprecursor fiber which has been prepared by the wet spinning process buthas a densified surface structure. In this patent, the densification ofa precursor fiber has been achieved by using a copolymer having aspecific composition and a coagulated fiber having specific properties,in combination with pressurized steam drawing. However, since noconsideration is given to the appropriate range of drawing conditionsafter coagulation, this process is unsatisfactory for the purpose ofpreparing a precursor fiber having a high degree of denseness and a highdegree of orientation. Moreover, since no mention is made of thestrength, elastic modulus, degree of crystal orientation, and degree ofvariation in tow fineness of the resulting precursor fiber, theproperties of a precursor fiber which are required for the formation ofa carbon fiber having excellent quality have been still unknown.Furthermore, it has been difficult to spin a precursor fiber stably at ahigh spinning speed of not less than 100 m per minute.

Thus, all conventional techniques have failed to provide a satisfactoryprecursor fiber for the formation of a high-quality and inexpensivecarbon fiber and a satisfactory process for preparing the same.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above-describedproblems of the prior art, and an object thereof is to provide anacrylonitrile-based precursor fiber for carbon fiber which has a highstrength, a high elastic modulus, a high degree of denseness, a highdegree of orientation, and a low degree of variation in tow fineness,and can hence be used to form a high-quality carbon fiber inexpensivelyby carbonizing for a shorter period of time, as well as a wet spinningprocess by which such an acrylonitrile-based precursor fiber for carbonfiber which has such properties can be rapidly and stably preparedwithout suffering fiber breakage frequently and without producing anyappreciable amount of fluff.

The present invention relates to an acrylonitrile-based precursor fiberfor carbon fiber which is prepared from an acrylonitrile-based copolymercontaining 96.0 to 98.5% by weight of acrylonitrile units, theacrylonitrile-based precursor fiber having a tensile strength of notless than 7.0 cN/dtex, an elastic modulus in tension of not less than130 cN/dtex, an iodine adsorption of not greater than 0.5% by weightbased on the weight of the fiber, a degree of crystal orientation (π) ofnot less than 90% as determined by wide-angle X-ray analysis, and adegree of variation in tow fineness of not greater than 1.0%.

The aforesaid acrylonitrile-based copolymer is preferably composed of96.0 to 98.5% by weight of acrylonitrile units, 1.0 to 3.5% by weight ofacrylamide units, and 0.5 to 1.0% by weight of carboxyl-containing vinylmonomer units.

In one embodiment of the present invention, the wet spinning process ispreferably employed as the method for spinning the acrylonitrile-basedprecursor fiber for a carbon fiber.

The present invention also relates to a process for preparing anacrylonitrile-based precursor fiber for a carbon fiber which comprisesthe steps of wet-spinning an acrylonitrile-based copolymer to form acoagulated fiber, subjecting the coagulated fiber to primary drawingcomprising in-bath drawing or a combination of in-air drawing andin-bath drawing, and subjecting thus obtained fiber to secondary drawinginvolving pressurized steam drawing, wherein the temperature of theheating roller located immediately before the introduction of the fiberinto a pressurized steam drawing device is adjusted to 120-190° C., thedegree of variation in steam pressure used in said pressurized steamdrawing is controlled so as to be not greater than 0.5%, and thecoagulated fiber is drawn in such a way that the proportion of thesecondary draw ratio to the overall draw ratio is greater than 0.2.

In one embodiment of the present invention, the overall draw ratio ispreferably not less than 13.

The present invention is more specifically described hereinbelow.

The acrylonitrile-based copolymer (which may hereinafter referred tosimply as the copolymer) used for the preparation of theacrylonitrile-based precursor fiber for a carbon fiber (hereinafterreferred to as the precursor fiber) in accordance with the presentinvention contains 96.0 to 98.5% by weight of acrylonitrile units asmonomer units. If the content of acrylonitrile units in the copolymer isless than 96% by weight, the fiber may undergo heat fusion in theoxidation step, so that the quality and performance of the carbon fibertend to be detracted from. Moreover, since the heat resistance of thecopolymer is reduced, filaments tend to adhere together during spinningof the precursor fiber, i.e., in the step of drying the fiber or thestep of drawing the fiber with a heating roller or pressurized steam. Onthe other hand, if the content of acrylonitrile units in the copolymeris greater than 98.5% by weight, the solubility of the copolymer insolvents is reduced and, therefore, the stability of the spinningsolution is detracted from. Moreover, the copolymer tends to makecoagulation fast, making it difficult to prepare dense precursor fiber.

Moreover, the copolymer used in the present invention preferablycontains 1.0 to 3.5% by weight of acrylamide units as monomer units.When the content of acrylamide units in the copolymer is 1.0% by weightor greater, the structure of the precursor fiber becomes sufficientlydense and, therefore, a carbon fiber having excellent performance isobtained. Moreover, the reactivity in the oxidation step is greatlyaffected by slight changes in copolymer composition. However, if thecontent of acrylamide units is 1.0% by weight or greater, a carbon fibercan be stably produced. Furthermore, it is believed that acrylamide hashigh random copolymerizability with acrylonitrile and, moreover, a heattreatment causes acrylamide to form ring structure in a manner verysimilar to that of acrylonitrile. In particular, acrylamide is much lesssusceptible to thermal decomposition in an oxidizing atmosphere, so thatit may be contained in larger amounts as compared withcarboxyl-containing vinyl monomers which will be described later.However, as the content of acrylamide units in the copolymer isincreased, the content of acrylonitrile units in the copolymer isdecreased and the heat resistance of the copolymer is reduced asdescribed previously. Accordingly, the content of acrylamide units issuitably not greater than 3.5% by weight.

Furthermore, the copolymer used in the present invention preferablycontains 0.5 to 1.0% by weight of carboxyl-containing vinyl monomerunits as monomer units. Usable carboxyl-containing vinyl monomersinclude, for example, acrylic acid, methacrylic acid and itaconic acid.If the content of carboxyl-containing vinyl monomer units is unduly low,the oxidation reaction is so slow that it become difficult to obtain ahigh-performance carbon fiber by oxidation for a short period of time.In order to carry out a oxidation treatment in a short period of time,the oxidation temperature must unavoidably be raised. Such hightemperatures tend to induce runaway reactions and may cause problemsfrom the viewpoint of processability and safety. On the other hand, ifthe content of carboxyl-containing vinyl monomer units is unduly high,the oxidation reactivity becomes so high that the region adjacent to thesurface of the fiber reacts rapidly during oxidation treatment, whilethe reaction of the central portion is retarded. Thus, the oxidizedfiber exhibits a zoning structure in a cross section thereof. With sucha structure, however, the central portion of the fiber in which theoxidized structure is underdeveloped cannot be prevented from beingdecomposed in the succeeding carbonization step at a higher temperature,resulting in a marked reduction in the performance (in particular,elastic modulus in tension) of the carbon fiber. This tendency becomesmore pronounced as the oxidation treatment time is reduced.

Furthermore, from the viewpoint of drawing in the spinning of theprecursor fiber and the performance of the carbon fiber, thepolymerization degree of the copolymer should preferably be such thatits limiting viscosity [η] is not less than 0.8. If the polymerizationdegree is unduly high, the solubility in solvents is reduced. Areduction in copolymer concentration tends to produce voids and cause areduction in drawing and spinning stability. For these reasons, it isusually preferable that its limiting viscosity [η] be not greater than3.5.

The precursor fiber of the present invention is formed from such acopolymer according to the wet spinning process, and has a tensilestrength of not less than 7.0 cN/dtex, an elastic modulus in tension ofnot less than 130 cN/dtex, an iodine adsorption of not greater than 0.5%by weight based on the weight of the fiber, a degree of crystalorientation (π) of not less than 90% as determined by wide-angle X-rayanalysis, and a degree of variation in tow fineness of-not greater than1.0%.

If the tensile strength of the precursor fiber is less than 7.0 cN/dtexor the elastic modulus in tension thereof is less than 130 cN/dtex, thecarbon fiber obtained from this precursor fiber has insufficientmechanical properties.

If the iodine adsorption of the precursor fiber is greater than 0.5% byweight, the denseness or orientation of the fiber structure is detractedfrom and the fiber becomes heterogeneous. This creates flaw during thecarbonizing step for converting the precursor fiber to a carbon fiber,and hence causes a reduction in the performance of the resulting carbonfiber. As used herein, the term “iodine adsorption” refers to the amountof iodine adsorbed to the fiber and serves as a measure of the degree ofdenseness of the fiber structure. Small values indicate that the fiberis denser.

If the degree of crystal orientation (π) of the precursor fiber is lessthan 90%, the precursor fiber shows a reduction in tensile strength andelastic modulus in tension, and the carbon fiber obtained from theprecursor fiber has insufficient mechanical properties. On the otherhand, in order to achieve a very high degree of crystal orientation (π),a higher draw ratio is required and this makes spinning processunstable. The range in which the precursor fiber can be easily preparedon an industrial basis is usually not greater than 95%.

As used herein, the term “degree of crystal orientation as determined bywide-angle X-ray analysis” is a measure of the degree of orientation ofthe copolymer molecular chains constituting the fiber in the directionof the fiber axis. From the half width (H) of circumferential intensitydistribution of diffraction points on an equatorial line of the fiber asrecorded by wide-angle X-ray analysis, the degree of orientation (π) canbe calculated according to the following equation:

Degree of orientation (π)=((180−H)/180)×100.

Moreover, if the degree of variation in tow fineness of the precursorfiber is greater than 1.0%, the resulting carbon fiber shows widevariation in tow weight per unit length, but also is likely to causeproblems such as an increase of defects responsible for breakage, areduction in tensile strength, and the creation of gaps betweenadjoining tows during the formation of a prepreg. As used herein, theterm “degree of variation in tow fineness” refers to the degree ofvariation determined by measuring the fineness of a tow consecutively inthe longitudinal direction.

Furthermore, the precursor fiber of the present invention preferably hasa surface roughness coefficient in the range of 2.0 to 4.0. Whenprecursor fibers have such a degree of surface roughness, the fusion ofthe fibers during oxidation treatment is suppressed, so that theyexhibit good processability during oxidation. Moreover, when theresulting carbon fibers are made into a composite material such asprepreg, the impregnation of the matrix resin into the void among carbonfibers is improved. Precursor fibers having a surface roughnesscoefficient within this range can be prepared by the wet spinningprocess. As used herein, the term “surface roughness coefficient” refersto a value obtained by using a scanning electron microscope to scan afiber with primary electrons in a direction perpendicular to the fiberaxis (i.e., in the direction of a fiber diameter), observing a curve ofsecondary (reflected) electrons reflected from the fiber surface, andcalculating l/d′ in which d′ is the diametral length of the central partof the fiber corresponding to 60% of the fiber diameter and l is thetotal length of the secondary electron curve in the range of d′(converted into the length of a straight line).

Now, the process for the preparation of a precursor fiber in accordancewith the present invention is described hereinbelow.

In order to prepare the acrylonitrile-based copolymer used in thepresent invention, there may be employed any of well-knownpolymerization techniques such as solution polymerization and slurrypolymerization. It is preferable to remove unreacted monomers,polymerization catalyst residues and other impurities from the resultingcopolymer to the utmost.

In the present invention, the aforesaid copolymer is wet-spun to form acoagulated fiber. Thereafter, this coagulated fiber is subjected toprimary drawing comprising in-bath drawing or a combination of in-airdrawing and in-bath drawing, and then to secondary drawing comprisingpressurized steam drawing.

In the wet spinning step, the aforesaid acrylonitrile-based copolymer isdissolved in a solvent to prepare a spinning solution. The solvent usedfor this purpose may be suitably selected from among well-known solventsincluding organic solvents such as dimethylacetamide, dimethyl sulfoxideand dimethylformamide; and aqueous solutions of inorganic compounds suchas zinc chloride and sodium thiocyanate.

Spinning is carried out by extruding the aforesaid spinning solutionthrough nozzle holes having a circular cross section into a coagulatingbath. An aqueous solution containing the solvent used for the spinningsolution is usually used as the coagulating bath.

Prior to drawing, the coagulated fiber thus obtained preferably has anelastic modulus in tension of 1.1 to 2.2 cN/dtex [dtex (decitex) is avalue based on the weight of the copolymer in the coagulated fiber]. Ifthe elastic modulus in tension of the coagulated fiber is less thanabout 1.1 cN/dtex, the fiber tends to be non-uniformly stretched in theinitial stages of the spinning process (e.g., in the coagulating bath),resulting in a variation in tow fineness and in the diameter offilaments within the tow. Moreover, since the various steps of thespinning process suffer a marked increase in drawing load and aconsiderable variation in drawing, it may become difficult to carry outcontinuous spinning stably.

On the other hand, if the elastic modulus in tension is greater thanabout 2.2 cN/dtex, filament breakage tends to occur in the coagulatingbath, and subsequent steps may suffer a reduction in drawing andstability. Consequently, it may become difficult to produce a highlyoriented fiber.

Such a coagulated fiber can be obtained by controlling the copolymercomposition, the solvent, the spinning nozzle, and the extrusion ratefrom the nozzle, and by regulating the concentration of the spinningsolution, the concentration and temperature of the coagulating bath, thespinning draft and the like so as to come within appropriate ranges.

Then, the coagulated fiber is subjected to primary drawing. In-bathdrawing may be carried out by drawing the coagulated fiber in thecoagulating bath or a drawing bath. Alternatively, the coagulated fibermay be partially drawn in air and then drawn in a bath. The in-bathdrawing is usually carried out in a hot water at 50-98° C., either in asingle bath or in two or more baths. The fiber may be washed before,after or during drawing.

After in-bath drawing and washing, the fiber is treated with a finishoil in the well-known manner, and then densified by drying. Thisdensification by drying needs to be carried out at a temperature higherthan the glass transition temperature of the fiber. In practice,however, this temperature may vary as the fiber is either in a moiststate or in a dry state. The densification by drying is preferablycarried out with a heating roller having a temperature of about 100 to200° C. For this purpose, one or more heating rollers may be used.

Thus, it is preferable that, after primary drawing, the fiber is treatedwith a finish oil and dried to a moisture content of not greater than 2%by weight (in particular, not greater than 1% by weight) by a heatingroller, and continuously subjected to secondary drawing involvingpressurized steam drawing. The reason for this is that the heatingefficiency of the fiber in pressurized steam is enhanced to permitdrawing in more compact equipment and that the development of phenomenadetracting from quality (e.g., the adhesion of filaments) can beminimized to cause a further improvement in the denseness and degree oforientation of the resulting fiber.

Next, the secondary drawing involving pressurized steam drawing isexplained. Pressurized steam drawing is a method comprising drawing afiber in an atmosphere of pressurized steam. This method not only canachieve a high draw ratio and hence permits stable spinning at a higherspeed, but also contributes to an improvement in the denseness anddegree of orientation of the resulting fiber.

In the present invention, it is important that, in the secondary drawinginvolving pressurized steam drawing, the temperature of the heatingroller located immediately before the pressurized steam drawing machineis adjusted to 120-190° C., and the degree of variation of steampressure in the pressurized steam drawing is controlled to be notgreater than 0.5%. This makes it possible to minimize variations in thedraw ratio applied to the fiber and in the ensuing variations in towfineness. If the temperature of the heating roller is lower than 120°C., the temperature of the acrylonitrile-based precursor fiber forcarbon fiber is not sufficiently raised to cause a reduction in drawing.

The secondary draw ratio is determined by the difference between thespeeds of the rollers located on the inlet and outlet sides of thepressurized steam drawing machine. In the present invention, the rollerlocated immediately before the pressurized steam drawing machine isusually a heating roller, and this may also serve as the final heatingroller for densification by drying. In the present invention, thesecondary drawing is two-stage drawing comprising drawing with theheating roller on the basis of the difference between the speeds of therollers located on the inlet and outlet sides of the pressurized steamdrawing machine, and drawing with pressurized steam.

The draw ratio imparted by the heating roller is determined by thetemperature of the heating roller and the drawing tension of the fiberin the secondary drawing. Consequently, the draw ratio imparted by theheating roller varies with drawing tension in the secondary drawing.Since the secondary draw ratio in a fixed period of time is always keptconstant by the difference between the speeds of the rollers located onthe inlet and outlet sides of the pressurized steam drawing machine, thedraw ratio imparted by pressurized steam varies with the draw ratioimparted by the heating roller. That is, the distribution between thedraw ratio imparted by the heating roller and the draw ratio imparted bypressurized steam varies.

In pressurized steam drawing, the appropriate treating time forachieving excellent drawing performance varies according to thetraveling speed of the fiber, steam pressure and the like. As thetraveling speed of the fiber become higher, and as steam pressurebecomes lower, a longer treating time is required. In the industrialproduction of precursor fibers, a treating length ranging from severaltens of centimeters to several meters is usually required. Moreover,since a section for preventing the leakage of steam is also required, atime lag occurs between drawing with the heating roller and drawing withpressurized steam. In a fixed period of time, the sum of the draw ratioimparted by the heating roller and the draw ratio imparted bypressurized steam remains constant. In actual equipment, however, bothtypes of drawing are not carried out concurrently. Consequently, thedraw ratio imparted to the fiber varies with the distribution betweendrawing with the heating roller and drawing with pressurized steam, andeventually causes variations in tow fineness.

For this reason, in order to suppress variations in the draw ratioimparted to the fiber, it is effective to minimize the time lag betweendrawing with the heating roller and drawing with pressurized steam.Accordingly, it is effective to make the length of the pressurized steamdrawing machine as small as possible. However, in order to heat thefiber sufficiently and secure industrially stable stretchability, thepressurized steam drawing machine needs to have a certain length. Thus,the prior art has not succeeded in avoiding variations in the draw ratioimparted to the fiber. The present inventors made investigation with aview to solving this problem, and have now revealed that, in order tosuppress variations in the draw ratio imparted to the fiber and hencevariations in the distribution between drawing with the heating rollerand drawing with pressurized steam, it is important to suppress the drawratio imparted by the heating roller and to minimize variations in thedrawing tension of the fiber in the secondary drawing.

As described previously, the draw ratio imparted by the heating rolleris determined by the temperature of the heating roller and the tensionproduced in the fiber by the secondary drawing. Accordingly, this can besuppressed by reducing the temperature of the heating roller and raisingthe pressure of steam used in the pressurized steam drawing. If thetemperature of the heating roller is unduly low, the heating efficiencyof the fiber in pressurized steam is reduced. Accordingly, the heatingroller is adjusted to a suitable temperature in the range of 130 to 190°C. Moreover, in order to allow the suppression of drawing with theheating roller and the features of pressurized steam drawing to beexhibited clearly, the pressure of steam used in the pressurized steamdrawing is preferably not less than 200 kPa·g (gauge pressure;hereinafter the same). Preferably, this steam pressure is suitablyregulated with consideration for the treating time. However, unduly highpressures may increase the leakage of steam. From an industrial point ofview, a steam pressure of not greater than about 600 kPa·g will suffice.

On the other hand, variations in the drawing tension of the fiber in thesecondary drawing can be suppressed by keeping the pressure of steamused in the pressurized steam drawing constant. Variations in thepressure of pressurized steam is preferably controlled so as to be notgreater than 0.5%. Moreover, it is also preferable to control theproperties of pressurized steam so that its temperature is not higherthan the saturated steam temperature at the pressure of interest byabout 3° C. and no water droplets are contained therein.

By determining the secondary drawing conditions in the above-describedmanner, it has first becomes possible to suppress variations in the drawratio imparted to the fiber, to carry out stable spinning at a high drawratio, and to increase the proportion of the secondary draw ratio to theoverall draw ratio. Especially in the case of high-speed spinning whichis carried out, for example, at a take-up speed of 100 m per minute andhence requires a high draw ratio, a high-quality precursor fiber can bestably prepared.

Moreover, in a preferred embodiment of the present invention, theproportion of the secondary draw ratio to age the overall draw ratio(secondary draw ratio/overall draw ratio) is greater than 0.2. In a morepreferred embodiment, the overall draw ratio is not less than 13. Thus,excellent spinning stability is achieved. As a result, even by employingthe wet spinning process, there can be obtained a precursor fiber havingexcellent tensile properties, a high degree of denseness, and a highdegree of orientation.

If the overall draw ratio is less than 13, the fiber cannot besufficiently oriented and, therefore, the denseness and degree oforientation of the resulting fiber are insufficient. Moreover, if thedraft in the coagulating bath is increased in order to compensate forthe decrease in draw ratio and thereby enhance productivity, filamentbreakage tends to occur owing to the high draft in the coagulating bath,and subsequent steps may suffer a reduction in stretchability andstability. If the overall draw ratio is unduly high, stable continuousspinning is difficult owing to increased drawing loads in the primarydrawing and the secondary drawing. Under ordinary conditions, theoverall draw ratio is preferably not greater than 25.

Moreover, in order to cause the pressurized steam drawing method tofully exhibit its high drawing capabilities and its characteristics inimproving the Sued denseness and degree of orientation of the fiber, theproportion of the secondary draw ratio to the overall draw ratio needsto be greater than 0.2. This can reduce drawing loads in the primarydrawing, so that no filament breakage occurs and, moreover, no reductionin stretchability or stability is caused in pressurized steam drawing.Consequently, there can be obtained a precursor fiber which issatisfactory with respect to all of denseness, mechanical properties,quality and production stability. These phenomena become more pronouncedas the spinning speed is increased. If the proportion of the secondarydraw ratio to the overall draw ratio is unduly high, the stability ofcontinuous spinning tends to be reduced owing to an increased load inthe secondary drawing. Accordingly, it is usually preferable that theproportion of the secondary draw ratio to the overall draw ratio be notgreater than 0.35.

When the carbon fibers obtained by carbonizing acrylonitrile-basedprecursor fibers for the formation of carbon fibers in accordance withthe present invention are arranged in one direction to form a prepreg,they can be made into a prepreg with about 30% higher productivity ascompared with conventional carbon fibers. The reason for this is thatthe acrylonitrile-based precursor fibers for the formation of carbonfibers and hence the carbon fibers have little longitudinal variation infineness and, therefore, the carbon fibers have little longitudinalvariation in openability.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is more specifically described with reference tothe following examples. In each of the examples and comparativeexamples, the copolymer composition, the limiting viscosity [η] of thecopolymer, the elastic modulus in tension of the coagulated fiber, thetensile strength and elastic modulus of the precursor fiber, the strandstrength and elastic modulus of the carbon fiber (abbreviated as CF inthe tables), the iodine adsorption, the degree of crystal orientation asmeasured by wide-angle X-ray analysis, the degree of variation in towfineness, the surface roughness coefficient, the moisture content of thefiber, and the degree of variation of steam pressure in pressurizedsteam drawing were determined according to the following methods.

(a) “Copolymer Composition”

This was determined by 1H-NMR spectroscopy (with a Nihon Denshi ModelGSZ-400 Superconducting FT-NMR).

(b) “Limiting Viscosity [η] of Copolymer”

This was measured by a dimethylformamide solution at 25° C.

(c) “Elastic Modulus in Tension of Coagulated Fiber”

A bundle of coagulated filaments was collected and quickly subjected toa tension test with a Tensilon in an atmosphere having a temperature of23° C. and a humidity of 50%. The test conditions included a samplelength (grip distance) of 10 cm and a pulling rate of 10 cm per minute.

The fineness. (dtex: the weight of the copolymer per 10,000 m of thecoagulated filament bundle) of the coagulated filament bundle wasdetermined according to the following equation, and the elastic moduluswas expressed in cN/dtex.

dtex=10,000×f×Qp/V

in which f is the number of filaments, Qp is the extrusion rate (g/min.)of the copolymer per nozzle hole, and V is the take-up speed (m/min.) ofthe coagulated fiber.

(d) “Tensile Strength and Elastic Modulus of Precursor Fiber”

A filament was collected and subjected to a tension test with a Tensilonin an atmosphere having a temperature of 23° C. and a humidity of 50%.The test conditions included a sample length (grip distance) of 2 cm anda pulling rate of 2 cm per minute.

The fineness (dtex: the weight per 10,000 m of the filament) of thefilament was determined, and the strength and the elastic modulus wereexpressed in cN/dtex.

(e) “Strand Strength and Elastic Modulus of Carbon Fiber”

These were measured according to the method described in JIS-7601.

(f) “Method for the Determination of Iodine Adsorption”

Two grams of precursor fibers were accurately weighed out and placed ina 100 ml Erlenmeyer flask. After 100 ml of an iodine solution (preparedby dissolving 100 g of potassium iodide, 90 g of acetic acid, 10 g of2,4-dichlorophenol, and 50 g of iodine in distilled water enough to makea total volume of 1,000 ml) was added thereto, the flask was shaken at60° C. for 50 minutes to carry out an iodine adsorption treatment.Thereafter, the fibers having undergone the adsorption treatment werewashed with ion-exchanged water for 30 minutes, further washed withdistilled water, and then dewatered by centrifugation. The dewateredfibers were placed in a 300 ml beaker. After the addition of 200 ml ofdimethyl sulfoxide, the fibers were dissolved therein at 60° C. Theamount of iodine adsorbed was determined by subjecting this solution topotentiometric titration using a 0.01 mol/l aqueous solution of silvernitrate.

(g) “Method for the Determination of the Degree of Crystal Orientationas Measured by Wide-angle X-ray Analysis”

This is a value obtained by recording diffraction points on anequatorial line of a polyacrylonitrile-based fiber by wide-angle X-rayanalysis, and calculating the degree of orientation (π) from the halfwidth (H) of the circumferential intensity distribution of thediffraction points according to the following equation.

Degree of orientation (π) (%)=((180−H)/180)×100

Wide-angle X-ray analysis (counter method):

(1) X-ray generator

RU 2000, manufactured by Rigaku Corp.).

X-ray source: CuKα (with a Ni filter).

Output: 40 kV, 190 mA.

(2) Goniometer

2155D1, manufactured by Rigaku Corp.).

Slit system: 2 MM, 0.5°×1°.

Detector: Scintillation counter

(h) “Degree of Variation in Tow Fineness”

In the longitudinal direction of a precursor fiber tow, the tow wasconsecutively cut to obtain 100 segments having a length of accurately 1m. After these segments were dried in a dryer at 85° C. for 12 hours,the dried weight of each segment was measured. The degree of variationwas determined according to the following equation.

Degree of variation (%)=(σ/E)×100

in which σ is the standard deviation of the measured data, and E is theaverage value of the measured date.

(i) “Method for the Determination of a Surface Roughness Coefficient”

First of all, the contrast conditions of a scanning electron microscopewere adjusted by using a magnetic tape as a standard sample.Specifically, using a high-performance magnetic tape as a standardsample, a secondary electron curve was observed under conditionsincluding an acceleration voltage of 13 kV, a magnification of 1,000diameters, and a scanning speed of 3.6 cm/sec. Thus, the contrastconditions were adjusted so that the average amplitude became equal toabout 40 mm. After this adjustment, a sample of a precursor fiber wasscanned with primary electrons in a direction perpendicular to the fiberaxis (i.e., in the direction of a fiber diameter). Using a line profileapparatus, a curve of secondary (reflected) electrons reflected from thefiber surface was displayed on the screen of a Brown tube andphotographed on a film at a magnification of 10,000 diameters. In thisstep, the acceleration voltage was 13 kV and the scanning speed was 0.18cm/sec.

The secondary electron photograph thus obtained was further printedwhile being enlarged twice (i.e., at an overall magnification of 20,000diameters). Thus, there was obtained a secondary electron curve diagram(photograph). A typical example thereof is shown in FIG. 1. In thisfigure, d is the fiber diameter, and d′ is the diametral length of theregion left after a 20% end part has been removed from each side of thefiber diameter (i.e., the diametral length of the central partcorresponding to 60% of the fiber diameter) and, therefore, d′=0.6d. lis the total length of the secondary electron curve in the range of d′(converted into the length of a straight line).

From the values of l and d′, the surface roughness coefficient can bedetermined by calculating l/d′.

(j) “Determination of Moisture Content of Fiber”

A fiber was dried in a dryer at 85° C. for 12 hours, and its weight W1before drying and its weight W2 after drying were measured. Its moisturecontent was determined according to the following equation.

Moisture content (%)=((W 1−W 2)/W 2)×100

(k) “Degree of Variation of Steam Pressure in Pressurized Steam Drawing”

During pressurized steam drawing, the pressure within the drawingmachine was monitored for 40 seconds. Pressure data were collected atintervals of 0.04 second, and the degree of variation was determinedaccording to the following equation.

Degree of variation (%)=(σ/E)×100

in which σ is the standard deviation of the measured data, and E is theaverage value of the measured date.

EXAMPLE 1

A copolymer composed of 97.1% by weight of acrylonitrile, 2.0% by weightof acrylamide, and 0.9% by weight of methacrylic acid and having alimiting viscosity [η] of 1.7 was dissolved in dimethylformamide toprepare a spinning solution having a copolymer concentration of 23% byweight. Using a nozzle having 12,000 holes, this spinning solution waswet-spun by extruding it into an aqueous solution of dimethylformamidehaving a concentration of 70% by weight and a temperature of 35° C. Theresulting coagulated fiber had an elastic modulus in tension of 1.59cN/dtex.

After the coagulated fiber was washed and desolvated in hot water whilebeing drawn at a draw ratio of 4.75, the fiber was dipped into a bath ofa finish containing silicone, and dried, collapsed by a heating rollerat 140° C. The resulting fiber had a moisture content of not greaterthan 0.1% by weight. Subsequently, the fiber was drawn in pressurizedsteam having a pressure of 294 kPa·g at a draw ratio of 2.8, and thendried again to obtain a precursor fiber. This precursor fiber was woundup at a speed of 100 m/min. During the pressurized steam drawing, thetemperature of the heating roller located immediately before thepressurized steam drawing machine was adjusted to 140° C., and thedegree of variation of steam pressure in the pressurized steam drawingwas controlled so as to be not greater than 0.2%. The steam supplied tothe pressurized steam drawing chamber was freed of water droplets bymeans of a drain trap, and the temperature of the pressurized steamdrawing chamber was adjusted to 142° C.

The overall draw ratio was 13.3, and the proportion of the secondarydraw ratio to the overall draw ratio was 0.21.

The control of steam pressure in the pressurized steam drawing wascarried out by installing JPG940A and BSTJ300 pressure transmitters(manufactured by Yamatake-Honeywell Corp.) in the drawing machine,sending the resulting data to a PID digital indicating controller(manufactured by Yokogawa Electric Corp.), and changing the opening ofan automatic pressure control valve according to instructions from theindicating controller.

In the spinning step, the breakage of filaments and the production offluff were seldom observed, indicating good spinning stability. Thisprecursor fiber had a tensile strength of 7.5 cN/dtex, an elasticmodulus in tension of 147 cN/dtex, an iodine adsorption of 0.2% byweight, a degree of orientation (π) of 93% by determined by wide-angleX-ray analysis, a degree of variation in tow fineness of 0.6%, and asurface roughness coefficient of 3.0.

Using a hot-air circulation oxidation oven, this fiber was heat-treatedin air at 230-260° C. under a 5% stretch for 30 minutes to form aoxidation fiber having a density of 1.368 g/cm³. Subsequently, thisfiber was subjected to a low-temperature heat treatment in an atmosphereof nitrogen at a maximum temperature of 600° C. under a 5% stretch for1.5 minutes. Then, using a high-temperature heat treatment oven having amaximum temperature of 1,400° C., it was further treated in the sameatmosphere under a −4% stretch for about 1.5 minutes. The resultingcarbon fiber had a strand strength of 4,800 MPa and a strand elasticmodulus of 284 GPa.

Comparative Examples 1-3

Spinning was carried out in the same manner as in Example 1, except thatthe coagulating bath comprised an aqueous solution of dimethylformamidehaving a concentration of 60% by weight and a temperature of 35° C.(Comparative Example 1), an aqueous solution of dimethylformamide havinga concentration of 73% by weight and a temperature of 35° C.(Comparative Example 2), or an aqueous solution of dimethylformamidehaving a concentration of 70% by weight and a temperature of 50° C.(Comparative Example 3).

In Comparative Example 1 much fluff was produced, and it was difficultto form a precursor fiber continuously. In Comparative Examples 2 and 3,the resulting precursor fiber was carbonized under the same conditionsas in Example 1. The elastic modulus in tension of the coagulated fiber,the amount of fluff, tensile strength, elastic modulus, iodineadsorption, and wide-angle X-ray degree of orientation of the precursorfiber, and the strand characteristics of the carbon fiber are shown inTable 1.

Comparative Examples 4 and 5

Spinning was carried out in the same manner as in Example 1, except thatthe conditions of pressurized steam drawing were altered. Specifically,the temperature of the heating roller located immediately before thepressurized steam drawing machine was 195° C., and the degree ofvariation of steam pressure in the pressurized steam drawing was about0.7% (Comparative Example 4), or the temperature of the heating rollerlocated immediately before the pressurized steam drawing machine was140° C., and the degree of variation of steam pressure in thepressurized steam drawing was about 0.7% (Comparative Example 5).

In Comparative Example 4, the degree of variation in tow fineness of theprecursor fiber was 1.7%, while in Comparative Example 5, the degree ofvariation in tow fineness of the precursor fiber was 1.2%.

EXAMPLES 2-4

The same acrylonitrile-based copolymer as used in Example 1 wasdissolved in dimethylacetamide to prepare a spinning solution having acopolymer concentration of 21% by weight. Using a nozzle having 12,000holes, this spinning solution was wet-spun by extruding it into anaqueous solution of dimethylacetamide having a concentration of 70% byweight and a temperature of 35° C.

Subsequently,. the resulting fiber was drawn in air at a draw ratio of1.5, and then washed and desolvated in hot water while being drawn.Thereafter, the fiber was dipped into a bath of a finish containingsilicone, and dried, collapsed by heating roller at 140° C.Subsequently, the fiber was drawn in pressurized steam having a pressureof 294 kPa·g, and then dried again to obtain a precursor fiber. Thisprecursor fiber was wound up at a speed of 100 m/min. During thepressurized steam drawing, the temperature of the heating roller locatedimmediately before the pressurized steam drawing machine was adjusted to140° C., and the degree of variation of steam pressure in thepressurized steam drawing was controlled so as to be not greater than0.2%. The steam supplied to the pressurized steam drawing chamber wasfreed of water droplets by means of a drain trap, and the temperature ofthe pressurized steam drawing chamber was adjusted to 142° C.

Furthermore, this fiber was carbonized under the same conditions as inExample 1 to obtain a carbon fiber. With respect to each example, theoverall draw ratio and the proportion of the secondary draw ratio to theoverall draw ratio, the elastic modulus in tension of the coagulatedfiber, the amount of fluff, tensile strength, elastic modulus, iodineadsorption, wide-angle X-ray degree of orientation, and degree ofvariation in tow fineness of the precursor fiber, and the strandcharacteristics of the carbon fiber are shown in Table 1.

Comparative Example 6

A precursor fiber was prepared under the same conditions as in Example2, except that the proportion of the secondary draw ratio to the overalldraw ratio was altered to the value shown in Table 1. Furthermore, thisfiber was fired under the same conditions as in Example 2 to obtain acarbon fiber. The elastic modulus in tension of the coagulated fiber,the amount of fluff, tensile strength, elastic modulus, iodineadsorption, and wide-angle X-ray degree of orientation of the precursorfiber, and the strand characteristics of the carbon fiber are shown inTable 1.

Comparative Examples 7-11

Precursor fibers were prepared and carbonized under the same conditionsas in Example 2, except that the composition of the acrylonitrile-basedcopolymer was altered as shown in Table 2. With respect to each example,the elastic modulus in tension of the coagulated fiber, the amount offluff, tensile strength, elastic modulus, iodine adsorption, andwide-angle X-ray degree of orientation of the precursor fiber, and thestrand characteristics of the carbon fiber are shown in Table 2. In thecase of comparative Example 7, the precursor fiber burned and fumed inthe oxidation step.

EXAMPLE 5

The same acrylonitrile-based copolymer as used in Example 1 wasdissolved in dimethylacetamide to prepare a spinning solution having acopolymer concentration of 21% by weight. Using a nozzle having 12,000holes, this spinning solution was wet-spun by extruding it into anaqueous solution of dimethylacetamide having a concentration of 70% byweight and a temperature of 35° C.

Subsequently, the resulting fiber was drawn in air at a draw ratio of1.5, and then washed and desolvated in hot water while being drawn.Thereafter, the fiber was dipped into a bath of a finish containingsilicone, and dried, collapsed by heating roller at 160° C.Subsequently, the fiber was drawn in pressurized steam having a pressureof 294 kPa·g, and then dried again to obtain a precursor fiber. Thisprecursor fiber was wound up at a speed of 140 m/min. During thepressurized steam drawing, the temperature of the heating roller locatedbefore the pressurized steam drawing machine was adjusted to 140° C.,and the degree of variation of steam pressure in the pressurized steamdrawing was controlled so as to be not greater than 0.2%. The steamsupplied to the pressurized steam drawing chamber was freed of waterdroplets by means of a drain trap, and the temperature of thepressurized steam drawing chamber was adjusted to 142° C.

Furthermore, this fiber was carbonized under the same conditions as inExample 1 to obtain a carbon fiber. The overall draw ratio and theproportion of the secondary draw ratio to the overall draw ratio, theelastic modulus in tension of the coagulated fiber, the amount of fluff,tensile strength, elastic modulus, iodine adsorption, wide-angle X-raydegree of orientation, and degree of variation in tow fineness of theprecursor fiber, and the strand characteristics of the carbon fiber areshown in Table 2.

EXAMPLE 6

Carbon fibers obtained in Comparative Example 4 were arranged inparallel so as to form a sheet having a carbon fiber basis weight of 125g/m². Two resin films (with a resin basis weight of 27 g/m²) wereprepared by applying #340 Epoxy Resin (manufactured by Mitsubishi RayonCo., Ltd.) to mold-releasing paper, and the above sheet was sandwichedtherebetween so that the epoxy resin came into contact with the carbonfibers. This assembly was passed through a prepreg production machine toproduce a prepreg having a basis weight of 125 g/m². As the productionspeed was gradually increased, the openability of carbon fibers wasreduced and about 1 mm wide splits including no carbon fiber came toappear at a frequency of 2-3 splits per 4-5 meters. The prepregproduction machine used in this example consisted of 7 pairs of heatedflat metallic press rolls, 1 pair of cooling rolls, and 1 pair of rubbertake-up rolls. When the carbon fibers sandwiched between the resin filmsprepared by applying an epoxy resin to mold-releasing paper was fedthereto, the resin was fluidized by heating on the surfaces of the pressrolls, and pressed so as to cause the resin to penetrate into the carbonfiber layer. Thereafter, the resulting prepreg was cooled and taken upby means of a pair of rubber rolls.

Then, the carbon fibers were replaced by carbon fibers obtained inExample 1. A prepreg having no split could be stably produced even at aproduction speed 30% higher than the production speed at which splitsappeared with the carbon fibers of Comparative Example 4.

Comparative Example 12

An acrylonitrile-based precursor fiber for carbon fiber was prepared inthe same manner as in Example 1, except that the temperature of theroller located before the pressurized steam drawing machine was adjustedto 115° C. This fiber produced much fluff and could not be easily woundup.

TABLE 1 Roller Degree of temperature variation of Elastic before steammodulus in Secondary pressurized pressure in Precursor fiber Copolymertension of draw steam pressurized Elastic composition, coagulatedOverall ratio/over drawing steam Tensile modulus in AN/AAM/MAA fiberdraw all draw machine drawing Amount of strength tension (wt. %)(cN/dtex) ratio ratio (° C.) (%) fluff (cN/dtex) (cN/dtex)) Example 197.1/2.0/0.9 1.59 13.3 0.21 140 ≦0.2 Little 7.5 147 2 ″ 1.94 13.3 0.26140 ≦0.2 Little 7.5 159 3 ″ 1.59 16.8 0.21 140 ≦0.2 Little 8.0 159 4 ″1.59 16.8 0.26 140 ≦0.2 Little 8.5 168 Comparative Example 197.1/2.0/0.9 3.88 13.3 0.21 140 ≦0.2 Much — — 2 ″ 1.06 13.3 0.21 140≦0.2 Moderately 6.4 132 much 3 ″ 3.44 13.3 0.21 140 ≦0.2 Moderately 5.7124 much 4 ″ 1.59 13.3 0.21 195 about 0.7 — — — 5 ″ 1.59 13.3 0.21 140about 0.7 — — — 6 ″ 1.94 13.3 0.17 140 ≦0.2 Moderately 6.6 128 muchPrecursor fiber CF stand Wide-angle Degree of Time for performanceIodine X-ray degree variation in flameproofing Elastic adsorption oforientation tow fineness treatment Strength modulus (wt. %) (π) (%) (%)(min.) (MPa) (GPa) Example 1 0.2 93.0 0.6 30 4800 284 2 0.2 93.5 0.6 304850 288 3 0.2 93.0 0.5 30 4750 286 4  0.15 94.0 0.6 30 5000 294Comparative Example 1 — — — — — — 2 0.5 89.0 — 30 4120 245 3 1.2 88.0 —30 4170 245 4 — — 1.7 — — — 5 — — 1.2 — — — 6 0.7 89.0 — 30 4120 268(Notes) AN: Acrylonitrile; AAM: Acrylamide; MAA: Methacrylic acid

TABLE 2 Roller Degree of temperature variation of Elastic before steammodulus in Secondary pressurized pressure in Precursor fiber Copolymertension of draw steam pressurized Elastic composition, coagulatedOverall ratio/over drawing steam Tensile modulus in AN/AAM/MAA fiberdraw all draw machine drawing Amount of strength tension (wt. %) *1)(cN/dtex) ratio ratio (° C.) (%) fluff (cN/dtex) (cN/dtex) ComparativeExample  7 99.0/0.5/0.5 3.18 13.3 0.26 140 ≦0.2 Much 7.1 150  894.0/5.0/1.0 1.32 13.3 0.26 140 ≦0.2 Little 6.4 135  9 97.0/1.0/2.0 2.2113.3 0.26 140 ≦0.2 Little 6.5 142 10 AN/2-HEMA/ 2.29 13.3 0.26 140 ≦0.2Little 6.7 156 MAA 97.0/2.0/1.0 11 AN/DAAM/ 2.03 13.3 0.26 140 ≦0.2Little 6.6 135 MAA 97.0/2.0/1.0 Example  5 AN/AAm/MAA 1.41 16.3 0.21 140≦0.2 Little 7.9 141 97.1/2.0/0.9 Precursor fiber CF stand Wide-angleDegree of Time for performance Iodine X-ray degree variation inflameproofing Elastic adsorption of orientation tow fineness treatmentStrength modulus (wt. %) (π) (%) (%) (min.) (MPa) (GPa) ComparativeExample  7 0.3 92.0 — 30 4210 255  8 1.4 90.0 — 30 4120 238  9 0.6 91.0— 30 4360 235 10 0.5 91.0 — 30 4310 247 11 0.8 90.5 — 30 4040 238Example  5 0.3 93.0 0.7 30 4610 289 (Notes) *1) AN/AAM/MAA is thecomposition the case where no constituent monomer units are indicatedAN: Acrylonitrile; AAM: Acrylamide; MAA: Methacrylic acid; 2-HEMA:2-Hydroxyethyl acrylate; DAAM: Diacetone acrylamide

EXPLOITABILITY IN INDUSTRY

According to the present invention, there is provided anacrylonitrile-based precursor fiber for carbon fiber which has a highstrength, a high elastic modulus, a high degree of denseness, a highdegree of orientation, and a low degree of variation in tow fineness,and can hence be used to form a high-quality carbon fiber inexpensivelyby carbonizing for a shorter period of time.

Moreover, according to the wet spinning process, an acrylonitrile-basedprecursor fiber for carbon fiber which has such properties can berapidly and stably prepared without suffering fiber breakage frequentlyand without producing any appreciable amount of fluff.

The acrylonitrile-based precursor fiber for carbon fiber in accordancewith the present invention has substantial uniformity of fineness in thelongitudinal direction, and the carbon fiber obtained by carbonizing ithas also substantial uniformity of fineness in the longitudinaldirection. This causes less variation of openability in the longitudinaldirection, so that this carbon fiber can be formed into prepregs withabout 30% higher productivity as compared with conventional carbonfibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a secondary electron curve diagram for the determination of asurface roughness coefficient.

DEFINITIONS OF CHARACTERS

d The fiber diameter.

d′ The diametral length of a central part of the fiber which

corresponds to 60% of the fiber diameter.

The total length of the secondary electron curve in the range of d′(converted into the length of a straight line).

What is claimed is:
 1. An acrylonitrile-based precursor fiber of anacrylonitrile-based copolymer containing 96.0 to 98.5% by weight ofacrylonitrile units, said acrylonitrile-based precursor fiber having atensile strength of not less than 7.0 cN/dtex, an elastic modulus intension of not less than 130 cN/dtex, an iodine adsorption of notgreater than 0.5% by weight based on the weight of the fiber, a degreeof crystal orientation (π) of not less than 90% as determined bywide-angle X-ray analysis, and, when made into a tow, a degree ofvariation in tow fineness of not greater than 1.0%.
 2. Anacrylonitrile-based precursor fiber as claimed in claim 1 wherein saidacrylonitrile-based copolymer is composed of 96.0 to 98.5% by weight ofacrylonitrile units, 1.0 to 3.5% by weight of acrylamide units, and 0.5to 1.0% by weight of carboxyl-containing vinyl monomer units.
 3. Anacrylonitrile-based precursor fiber as claimed in claim 1 which has beenformed by the wet spinning process.
 4. A process for preparing anacrylonitrile-based precursor fiber as claimed in claim 1, whichcomprises the steps of wet-spinning an acrylonitrile-based copolymer toform a coagulated fiber, subjecting the coagulated fiber to primarydrawing comprising in-bath drawing or a combination of in-air drawingand in-bath drawing, and subjecting thus obtained fiber to secondarydrawing involving pressurized steam drawing, wherein the temperature ofthe heating roller located immediately before the introduction of thefiber into a pressurized steam drawing machine is adjusted to 120-190°C., the degree of variation of steam pressure in said pressurized steamdrawing is controlled so as to be not greater than 0.5%, and thecoagulated fiber is drawn in such a way that the proportion of thesecondary draw ratio to the overall draw ratio is greater than 0.2.
 5. Aprocess for preparing an acrylonitrile-based precursor fiber for carbonfiber as claimed in claim 4 wherein the overall draw ratio is not lessthan
 13. 6. A process for preparing an acrylonitrile-based precursorfiber for carbon fiber as claimed in claim 4 wherein saidacrylonitrile-based copolymer is composed of 96.0 to 98.5% by weight ofacrylonitrile units, 1.0 to 3.5% by weight of acrylamide units, and 0.5to 1.0% by weight of carboxyl-containing vinyl monomer units.
 7. Aprocess for preparing an acrylonitrile-based precursor fiber for carbonfiber as claimed in claim 4, wherein, prior to drawing, the coagulatedfiber has an elastic modulus in tension of 1.1 to 2.2 cN/dtex.
 8. Aprocess for preparing an acrylonitrile-based precursor fiber for carbonfiber as claimed in claim 4 wherein said pressurized steam drawing iscarried out at a steam pressure of not less than 200 kPa (gaugepressure).
 9. A process for preparing an acrylonitrile-based precursorfiber for carbon fiber as claimed in claim 4 wherein the fiber subjectedto said pressurized steam drawing has a moisture content of not greaterthan 2% by weight.
 10. A carbon fiber formed by oxidation andcarbonizing an acrylonitrile-based precursor fiber for carbon fiber asclaimed in claim
 1. 11. An acrylonitrile-based precursor fiber forcarbon fiber as claimed in claim 2 which has been formed by the wetspinning process.
 12. A process for preparing an acrylonitrile-basedprecursor fiber for carbon fiber as claimed in claim 5 wherein saidacrylonitrile-based copolymer is composed of 96.0 to 98.5% by weight ofacrylonitrile units, 1.0 to 3.5% by weight of acrylamide units, and 0.5to 1.0% by weight of carboxyl-containing vinyl monomer units.
 13. Theprocess for preparing an acrylonitrile-based precursor fiber for carbonfiber as claimed in claim 12 wherein, prior to drawing, the coagulatedfiber has an elastic modulus in tension of 1.1 to 2.2 cN/dtex.
 14. Theprocess for preparing an acrylonitrile-based precursor fiber for carbonfiber as claimed in claim 13 wherein said pressurized steam drawing iscarried out at a steam pressure of not less than 200 kPa (gaugepressure).
 15. The process for preparing an acrylonitrile-basedprecursor fiber for carbon fiber as claimed in claim 14 wherein thefiber subjected to said pressurized steam drawing has a moisture contentof not greater than 2% by weight.
 16. A carbon fiber formed by oxidationand carbonizing an acrylonitrile-based precursor fiber for carbon fiberas claimed in claim
 2. 17. A carbon fiber formed by oxidation andcarbonizing an acrylonitrile-based precursor fiber for carbon fiber asclaimed in claim
 3. 18. A tow consisting of acrylonitrile-basedprecursor fibers prepared from an acrylonitrile-based copolymercontaining 96.0 to 98.5% by weight of acrylonitrile units, saidacrylonitrile-based precursor fiber having a tensile strength of notless than 7.0 cN/dtex, an elastic modulus in tension of not less than130 cN/dtex, an iodine adsorption of not greater than 0.5% by weightbased on the weight of the fiber, a degree of crystal orientation (π) ofnot less than 90% as determined by wide-angle X-ray analysis, and adegree of variation in tow fineness of not greater than 1.0%.
 19. A towas claimed in claim 18 wherein said acrylonitrile-based copolymer iscomposed of 96.0 to 98.5% by weight of acrylonitrile units, 1.0 to 3.5%by weight of acrylamide units, and 0.5 to 1.0% by weight ofcarboxyl-containing vinyl monomer units.
 20. A tow as claimed in claim18 which has been formed by a wet spinning process.
 21. A tow as claimedin claim 19 which has been formed by a wet spinning process.
 22. Anacrylonitrile-based precursor fiber of an acrylonitrile-based copolymercontaining 96.0 to 98.5% by weight of acrylonitrile units, saidacrylonitrile-based precursor fiber having a tensile strength of notless than 7.0 cN/dtex, an elastic modulus in tension of not less than130 cN/dtex, an iodine adsorption of not greater than 0.5% by weightbased on the weight of the fiber, a degree of crystal orientation (π) ofnot less than 90% as determined by wide-angle X-ray analysis.
 23. Anacrylonitrile-based precursor fiber as claimed in claim 22 wherein saidacrylonitrile-based copolymer is composed of 96.0 to 98.5% by weight ofacrylonitrile units, 1.0 to 3.5% by weight of acrylamide units, and 0.5to 1.0% by weight of carboxyl-containing vinyl monomer units.
 24. Anacrylonitrile-based precursor fiber as claimed in claim 22 which hasbeen formed by a wet spinning process.
 25. An acrylonitrile-basedprecursor fiber as claimed in claim 23 which has been formed by a wetspinning process.